Subtitles, audio descriptions and signed broadcasts are likely to remain unavailable to viewers of Channel 4 until mid-November, after an incident in September severely affected the broadcaster’s output.

A month on from a fault at Red Bee Media’s west London headquarters, which also led to problems with the transmission of BBC and Channel 5 shows, accessible programming remains unavailable. More than 500 complaints have been logged by Ofcom.

In its latest statement, Channel 4 apologised for the issue, saying it realised it was “incredibly frustrating” for viewers and engineers were working “around the clock” to resume normal service. However, owing to the scale of underlying technical issues, it said subtitles and other accessibility services may not be available until the middle of next month.

In the incident on 25 September, channels including Channel 4, More4, Channel 5 and S4C were taken off air. BBC One and BBC Two were also affected but they were able to switch to a backup.

It was reported that the activation of a fire alarm at Red Bee’s broadcast centre in White City, west London, led to server outages. In its statement, Channel 4 clarified that “the fire suppression system was triggered … As a result a large number of hard disks in a variety of systems were severely damaged. This significantly impacted the playout servers, which meant our channels and on-demand services were temporarily taken off air.”

Although the broadcaster was able to resume scheduled broadcasting the following day, technical faults have continued to hinder programming. E4 was particularly affected on 30 September, when an episode of Married at First Sight broadcast the evening before was accidentally repeated, in place of the series finale.

The subsequent lack of accessibility measures has angered many viewers with sight and hearing problems, who have been unable to watch their favourite shows. Mark Atkinson, the chief executive of the Royal National Institute for Deaf People, said television was “a key part of our culture, the basis for everyday conversation with friends, family and colleagues.

“But for more than three weeks, the 12 million people in the UK who are deaf or have hearing loss have felt excluded and increasingly angry because the system to provide subtitles and signed content is broken. It’s impossible to imagine a failure that affected the hearing community being allowed to go on for so long.

“The BBC and Channel 5 are now offering a near-normal service, but it is unacceptable that the system could have failed so spectacularly, and that Channel 4 have still not fixed the problem. [Furthermore] there was a failure across the board to communicate to deaf people regularly and – most importantly – accessibly.

“We’re pleased that since we met with them last week, Channel 4 have started providing updates in British Sign Language to the deaf community. They must ensure deaf people and people with hearing loss are kept informed about what steps they are taking until the problem is fixed.”

The deaf journalist and campaigner Liam O’Dell said his reaction to the latest news from Channel 4 was mixed. “It’s welcome in the sense that after weeks of deaf people calling out for a timeframe for access services to be reinstated, we now have a rough estimate. The concern, however, is that it will be around two months after the initial incident took place, meaning the amount of shows deaf viewers will need to find time to catch up on will be overwhelming.

“After weeks of poor communication from Channel 4, I hope they will now provide regular updates on their progress towards this target, to reassure deaf viewers like me who are rightly frustrated and distressed.”

Channel 4 said it was beginning to introduce subtitles to some programmes on its on-demand service All 4 this week, but this was “a very labour-intensive process, so it isn’t possible to make every programme available simultaneously … We know this isn’t good enough, but it’s a start.”